U.S. patent number 4,529,876 [Application Number 06/566,065] was granted by the patent office on 1985-07-16 for integrated optics transducer.
Invention is credited to Clifford G. Walker.
United States Patent |
4,529,876 |
Walker |
July 16, 1985 |
Integrated optics transducer
Abstract
A transducer utilizes a laser source, photoelastic waveguides,
two optical beam paths and a detector for processing optical energy
from the laser through a stress transfer medium and thereby detects
stress forces present on the transducer. This allows forces such as
pressure, strain, voltage, or current to be detected and converted
from one form to another for measurement of the force and response
thereto. Input forces are detected as optical frequency shifts and
converted to electrical signal outputs for indicating circuitry. In
an integrated optic format, the transducer package volume is small,
allowing ready use in guidance or navigation systems. Light
generated by the laser travels along two paths, is optically
stressed by the force transfer member and is combined with a
reference signal to obtain the stress intelligence.
Inventors: |
Walker; Clifford G.
(Huntsville, AL) |
Family
ID: |
24261328 |
Appl.
No.: |
06/566,065 |
Filed: |
December 27, 1983 |
Current U.S.
Class: |
250/227.19;
356/477; 356/35.5 |
Current CPC
Class: |
G01D
5/35303 (20130101); G02F 1/0134 (20130101); G01L
1/243 (20130101) |
Current International
Class: |
G01L
1/24 (20060101); G01D 5/353 (20060101); G01D
5/26 (20060101); G02B 005/14 () |
Field of
Search: |
;73/657,800 ;356/350
;250/227,231G,231R ;350/96.13,96.14 ;455/616,610-612 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Davis et al., Techniques for Shot Noise Limited Inertial Rotation
Measurement Using a Multiturn Fiber Sagnac Interferometer, SPIE 161
157, 1978, p. 131, Laser Inertial Rotation Sensors. .
Goss et al., "Fiber-Optic Rotation Sensor Technology" from Applied
Optics, vol. 19, No. 6, Mar. 15, 1980, pp. 852-858. .
Merz et al., "GaAs Integrated Optical Circuits by Wet Chemical
Etching" from IEEE Journal of Quantum Electronics, vol. OE-15, No.
2, Feb. 1979, pp. 72-82. .
Garmire, "Optical Waveguide for Laser Gyro Applications" from SPIE
vol. 157, Laser Inertial Rotation Sensors, 1978, pp. 95-99. .
Leonberger et al., "Low-Loss GaAs p.sup.+ n.sup.- n.sup.+
Three-Dimensional Optical Waveguides" from Applied Physics Letters,
vol. 28, No. 10, May 15, 1976, pp. 616-619. .
Anderson, "Integrated Optical Spectrum Analyzer: An Imminent
`Chip`", IEEE Spectrum, Dec. 1974, pp. 22-29..
|
Primary Examiner: Nelms; David C.
Assistant Examiner: Brophy; J. Jon
Attorney, Agent or Firm: Lane; Anthony T. Gibson; Robert P.
Bush; Freddie M.
Government Interests
DEDICATORY CLAUSE
The invention described herein may be manufactured, used, and
licensed by or for the Government for governmental purposes without
the payment to me of any royalties thereon.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a division of applicant's copending application
Ser. No. 371,867, filed Apr. 26, 1982, which issued June 12, 1984
as U.S. Pat. No. 4,454,418.
Claims
I claim:
1. A transducer comprising: a laser for generating a beam of
coherent light, a photodetector, photoelastic waveguide means
disposed between said laser and said photodetector for providing
first and second optical paths therebetween, said photodetector
being aligned for detecting light passing through said first and
second optical paths, stress transfer means disposed adjacent said
waveguide means for subjecting said first path to stress forces,
said second optical path being unstressed, signal processing means
adapted to receive an input from said photodetector for providing a
signal output in response to stress forces on said stress transfer
means, and a laser modulator for driving said laser at a particular
frequency rate and for providing said driving rate to said signal
processing means as a reference signal, said laser, waveguide, and
photodetector being aligned for directing said light within a plane
from said laser to said photodetector, and wherein said
photoelastic waveguide means comprises an input waveguide, an
output waveguide, and first and second adjacent waveguides for
providing said first and second optical paths, said waveguide being
coupled between the input and output waveguides for providing
waveguide-to-waveguide directional coupling between said input
waveguide and said first and second waveguides, and providing said
directional coupling between said first and second waveguides and
said output waveguide, said input waveguide being disposed for
receiving the light output from said laser, and said output
waveguide being disposed for coupling light passed through said
first and second waveguides to said photodetectors; and wherein
said waveguide means is planar for providing a plane of travel for
laser light, said waveguide means, said laser, and said
photodetector are a monolithic substrate of integrated optics.
2. A transducer as set forth in claim 1 wherein said stress
transfer means is disposed for applying forces to said first
optical path that are parallel to the plane of said waveguide
means.
3. A transducer as set forth in claim 1 wherein said stress
transfer means comprise an electro-mechanical stress producing
member and an electrical signal converter for converting electrical
input signals to an incremental, variable voltage output, said
stress producing member being responsive to said voltage output for
providing a mechanical force to said first waveguide for stressing
said first waveguide.
4. An integrated optics transducer comprising: a laser for
generating an output coherent light beam; a photodetector having an
input and an output; photoelastic waveguide means having an input,
an output, and first and second separate optical paths
therebetween; said waveguide means having the input coupled to
receive said laser beam output and having the output coupled to
said photodetector input; said photodetector providing an
electrical signal output indicative of and in response to light
input received from said waveguide means; stress transfer means
disposed adjacent said photoelastic waveguide means for subjecting
a portion of the waveguide means to stress forces for affecting a
portion of light energy passing therethrough; said waveguide means
being planar and providing a plane of travel for light energy, and
wherein said laser, said photodetector and said photoelastic
waveguide means are an integrated optics structure.
5. An integrated optics transducer as set forth in claim 4 wherein
said photoelastic waveguide means has an input optical waveguide
that functions as said input, an output optical waveguide that
functions as said output, and first and second adjacent optical
waveguides coupled between the input and output waveguides and
functioning as said first and second optical paths between said
input and output for dividing input light signals substantially
equally between said first and second waveguides and for bringing
said divided light signals back together at said output waveguide
for coupling to said photodetector input; and said stress transfer
means being disposed for subjecting only said first optical
waveguide to said stress forces in response to stress forces on
said stress transfer means, said second optical waveguide being
unstressed.
6. An integrated optics transducer as set forth in claim 5 wherein
said stress transfer means is disposed for transferring forces to
said first optical waveguide that are normal to the plane of travel
of laser light within said waveguide means; and wherein said
integrated optics structure is a monolithic substrate.
Description
SUMMARY OF THE INVENTION
An integrated optics transducer circuit for phase detection
produces an output beam that is modified in proportion to a change
in magnitude of input stress energy such as from pressure, strain,
voltage, or current. For phase detection a reference input beam has
the intensity divided and coupled into two waveguides. One
waveguide is subjected to stress, the other is unstressed. The
beams pass through the waveguides, are recombined, and are detected
by a photodetector which provides an electrical output indicative
of any phase difference between the beams passing through the two
waveguides. The phase difference is proportional to a change in
stress input.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic view of the phase detection transducer
using integrated optics and electronics.
FIGS. 2A, 2B, and 2C are diagrammatic cross-sectional views of the
waveguide assembly, showing various stress or force transfer
members coupled to the waveguide.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Generally an output laser beam is phase modified and detected by a
detector in proportion to the change in magnitude of the input
forces, pressure, strain, voltage or current. For phase detection
the beam is split, a first part of the beam being directed along a
first path toward a detector, and a second part of the beam being
directed along a second path toward the same detector. One beam
passes through an unstressed photoelectric electro-optic waveguide
and the other beam passes through a stressed photoelastic
electro-optic waveguide. The phase difference between the stressed
and unstressed waveguides is proportional to a change in input. The
two beams exit the waveguides and are coupled to the detector where
an interference condition is established.
Referring now to the drawings wherein like numbers represent like
parts, a phase detection transducer is shown in FIG. 1. A laser
source 10, input waveguide 11, a signal waveguide 40, reference
waveguide 42 and an output waveguide 44 comprises the optical
circuitry. A photodetector 20 and signal processor 50 comprise the
electrical circuitry. Output signals from the signal processor may
be coupled to any monitoring system such as a visual display or a
computer. The transducer can be operated in an open or closed loop
mode where the signal processor is typically a demodulator with an
output proportional to the phase difference between the beams.
A stress transfer element 28, as shown in FIGS. 1 and 2A-2C, is
mounted on electro-optical waveguide 40 and responds to forces such
as pressure, strain, voltage or current to stress the waveguide.
Substrate 26, which contains the waveguides and other optical
circuit portions therein, may be a gallium arsenide or other
similar material used in integrated optic systems. Transfer element
28 is disposed for transferring forces to waveguide 40 that are
unidirectional to the plane of travel of laser light within the
waveguide.
In the diagrammatic view of FIG. 2A waveguides 40 and 42 are shown
formed in substrate 26 and stress transfer element 28A is fixedly
attached to the surface of waveguide 40 for applying pressure
forces normal thereto. Pressure or force .DELTA.p is applied only
to waveguide 40 and normal to the surface of the substrate. Thus,
element 28A applies force normal to the plane of travel of laser
light within waveguide 40.
FIG. 2B discloses stress transfer element 28B to be disposed across
a portion of the surface 31 of substrate 26 covering and including
part of the waveguide 40 surface for measuring strain, strain
forces .DELTA..epsilon. being applied in a plane parallel with the
plane of surface 31 of substrate 26 and parallel to the plane of
waveguide 40. Strain forces are shown typically as symbolized by
the x's above transfer element 28B. The x's represent strain forces
applied to element 28B and directed normally into the paper along
the length of the element since this allows maximum strain signal
transfer along the length of the waveguide 40. Transverse forces or
forces at an angle will be sensed in the waveguide but maximum
sensitivities occurs when forces are oriented longitudinally.
FIG. 2C discloses a transfer element or stress producing member 28C
such as an electro-optic element attached to waveguide 40, which
receives a voltage input .DELTA.V from a converter 34 and responds
with a mechanical force output, stressing waveguided 40. Converter
34 may be a current-to-voltage converter or a
voltage-to-voltage-converter for responding to the desired input
.DELTA.i or .DELTA.v to provide the representative .DELTA.V
output.
The externally applied energy in the form of pressure, strain,
current, or voltage forces stresses waveguide 40 in compression or
tension depending on the direction of the forces, and thereby
changes the optical path length of the waveguide.
In open-loop operation of the integrated optics transducer
structure of FIG. 1, the laser beam is coupled between adjacent
waveguides 40 and 42 using waveguide-to-waveguide directional
coupling (W-W-DC) methods as is well known in the art. The
horizontally polarized laser beam from source 10 has an output
frequency f.sub.0 polarized perpendicular to the direction of
stress forces on waveguide 40. The laser beam has intensity I which
travels in input waveguide 11 prior to being split via W-W-DC into
two beams with intensity I/.sub.2. One beam travels in the signal
waveguide 40 which is subject to input forces from stress transfer
element 28 that stresses the waveguide, changing its optical path
length (OPL). The other beam continues to travel in the reference
waveguide 42 and passes through an electro-optic phase bias element
46. The bias voltage for element 46 is supplied by power supply 48,
providing a 90.degree. phase lag between the two beams when there
is no input stress. The detection of small stresses in waveguide 40
are more easily detected because the beams are separated by
90.degree., which is the maximum slope point. The two beams are
added in waveguide 44 and the resulting interference signal is
detected by detector 20. The output of detector 20 is coupled to a
signal processor or demodulator 50 where the transducer output is
developed.
In closed-loop operation, the signal processor 50 generates a
feedback signal 49 to the D.C. power supply 48 which produces a
voltage to the phase modulator 46 that maintains a 90.degree.
separation between beams. In this case the voltage change coupled
to the phase modulator 46 is proportional to the input stress of
waveguide 40. In this mode of operation Equation (10) still applies
but a null will be maintained at .DELTA..phi. equal zero so that a
change in laser intensity I will not affect the accuracy of the
measurement. In applications where this feedback is not desired the
transducer is operated in an open-loop mode by simply omitting
feedback signal 49 and .DELTA..phi. is not equal to zero.
Output signals from processor 50 may be coupled to any monitoring
system such as a visual display or a computer. The laser modulator
52 drives laser 10 and also provides a reference signal to
processor 50, providing detector shot-noise-limited operation.
With a stress input applied through members 28 (28A, 28B, or 28C)
to the wavesguide, the index of refraction of the waveguide is
changed, generating a phase change in beam intensity output coupled
from waveguide 40 to waveguide 44. This phase changed output
combines with the unchanged beam intensity passed through waveguide
42 to provide interference fringes indicative of the amount of
phase shift, which is itself indicative of the degree of stress
input.
For the structure of FIG. 1, the stress transfer element 28 is
similar to the particular stress transfer elements shown in FIGS.
2A-2C of applicant's U.S. Pat. No. 4,454,418, issued June 12, 1984,
depending on the particular stress being measured. The primary
difference is that, in the phase detection transducer, stress is
applied to only one portion of the waveguide 40 and that is
substantially along a straight line. Alternatively, in the
disclosure of U.S. Pat. No. 4,454,418, the stress transfer members
are circular, covering the entire surface over the resonator
16.
The phase change .DELTA..phi. between the beams can be derived as
follows:
wherein .DELTA.n is the change in index of refraction caused by
stress, L is the length of waveguide being stressed, and .lambda.
is the wavelength of the laser beam. To measure pressure:
where B is the waveguide photoelastic constant in Brewsters
(10.sup.-13 cm.sup.2 /dyne) and .DELTA.P is the change in pressure
(dyne/cm.sup.2). Using equations (1) and (2):
To measure strain:
where E is the waveguide modulus of elasticity (dyne/cm.sup.2) and
.DELTA..epsilon. is a change in unit strain (cm/cm). Using
equations (1) and (4):
To measure voltage (v) or current (i), the change in index .DELTA.n
is as follows:
where K.sub.1 and K.sub.2 are constants which are a function of the
converter and the electro-optic transfer material
(.DELTA.n/.DELTA.V). From equation (1):
The equations for beam intensity at the detector are:
where
I.sub.o =intensity at detector and
I=laser beam intensity.
For a 90.degree. phase lag:
The primary goal of integrated optics is to integrate a variety of
discrete optical elements, both active and passive, into a
monolithic, miniaturized planar structure. It is an optical analogy
of integrated circuit technology in the electronics industries. By
this analogy, it is reasonable to assume that optical systems in
the integrated optics configuration can be made more efficient,
compact and stable at lower cost. In the embodiment shown, which is
a monolithic structure, gallium-arsenide is used as the substrate.
Other substrates may be resorted to such as silicon,
lithium-niobate, or fiber optic waveguides however, in these
structures an interface must be used between the laser and the
substrate and the detector.
Obviously many modifications and variations of the present
invention are possible in the light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims the invention may be practiced otherwise than as
specifically described.
* * * * *